In the fields of security, safety, etc., infrared cameras, infrared sensors, and the like are used as, for example, crime prevention devices and authorization devices. These sensors use infrared rays, and optical elements used in the sensors comprise infrared-transmitting materials that allow infrared rays to pass through. More specifically, infrared-transmitting materials that allow infrared rays having a wavelength of 3 to 5 μm and 8 to 12 μm, which are called “atmospheric windows,” to pass through are required.
These devices have recently been increasingly required to have high performance, a small size, and high versatility, due to, for example, the heightened awareness of security, safety, etc., and social needs. Therefore, it is also necessary to reduce the size of sensors used in these devices. Optical elements are required to have high performance and a small size, and high productivity is required for the production process of optical elements.
Examples of infrared-transmitting materials include germanium (Ge) and zinc selenide (ZnSe). However, because they are crystals, the processing means of these infrared-transmitting materials is limited to polish forming. Therefore, it is difficult, in terms of process and cost, to mass-produce optical elements having a complicated shape, such as aspheric lenses and lens arrays, using these materials. In particular, it is not easy to use germanium, which is an expensive material, in versatile sensors and the like.
In contrast, examples of infrared-transmitting materials that are not crystals include chalcogenide glasses comprising S, Se, Te, As, etc., as main components. Various proposals have been made on glasses suitable for mold forming and glasses having high glass-forming ability, which are advantageous for the mass production of optical elements (NPL 1, etc.). However, the chalcogenide glasses disclosed in NPL 1, etc., contain many highly toxic elements, such as Se and As. Thus, these glasses have safety concerns.
Moreover, PTL 1 discloses an infrared-transmitting glass comprising two or more materials selected from Groups III, V, VI, and VII of the periodic table in preselected amounts for forming a low-dispersion glass at wavelengths of infrared energy. However, the Examples of PTL 1 specifically disclose only a glass containing Se, which has safety concerns. The specific composition of a glass further containing S of Group VI is nowhere described in PTL 1.
In order to solve the above concerns, PTL 2 discloses an “infrared-transmitting glass for mold forming, the glass comprising, in terms of molar concentration, 2 to 22% of Ge, 6 to 34% of at least one element selected from the group consisting of Sb and Bi, 1 to 20% of Sn, and 58 to 70% of at least one element selected from the group consisting of S, Se and Te.” According to the Ge—Sb—Sn—S glass of PTL 2, an infrared-transmitting glass that is suitable for mold forming without containing highly toxic elements, such as Se and As, can be obtained (claim 1, Advantageous Effects of Invention, etc.).
However, the infrared transmission limit wavelength of the infrared-transmitting glass of PTL 2 on the long wavelength side is about 11 μm, and this glass cannot sufficiently cover atmospheric windows. There is room for improvement in this respect.
Furthermore, prior-art documents relating to the present application include NPL 2 to NPL 4; however, these documents only disclose Ga—Sb—S three-component glasses, but do not disclose the production of glasses comprising four or more components by adding other components, and the effects of such glasses.